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Cholera toxin, molecular model. Cholera toxin (choleragen) is a protein complex secreted by the bacterium Vibrio cholerae, which causes the disease cholera. It is made up of a wedge-shaped A subunit (right) and five B subunits (left) that form a ring which binds to the intestinal lining. In its most severe forms, cholera is one of the most rapidly fatal illnesses known.